Remote current detector

ABSTRACT

A remote current sensor for use in a fault detector for an electrical transmission system. The remote current sensor includes a Hall effect transducer for measuring the magnetic field surrounding a conductor of the transmission system to thereby monitor the currents flowing through the conductor. The transducer is positioned between a pair of tapered pole pieces for concentrating low density magnetic flux in the air into a high density magnetic flux focused onto the transducer. The pole pieces enable the sensors to be positioned a distance from the conductor which is greater than the electrical arcing distance so as to allow use of the air gap between the conductor and sensor as insulation.

This is a division of application Ser. No. 239,344 filed Mar. 2, 1981now U.S. Pat. No. 4,408,155.

BACKGROUND OF THE INVENTION

This invention relates to a remote current sensor for use in a faultdetector for electrical transmission system. The remote current sensormeasures the magnetic field surrounding a conductor of the transmissionsystem to monitor the current flowing through the conductor. The remotecurrent sensor is positioned at a location sufficiently distant from theconductor to prevent electrical arcing from occurring without the use ofan insulating material. The fault detector responds to the output of theremote current sensor to detect a fault within one quarter of the faultcurrent cycle.

Fault detectors have been known in which a current transformer is usedto monitor the current flowing through a conductor of an electricaltransmission system. A current transformer typically requires anexpensive insulating material such as porcelain to protect thetransformer from electric fields. Fault detectors have also used a Halleffect transducer to monitor the current flowing through a conductor butalone the transducer is ineffective at distances from the conductorgreater than the electrical arcing distance. At distances sufficientlyclose to the conductor to provide a meaningful measurement, the Halleffect transducer requires the same insulation as the currenttransformer to protect the transducer from electric fields.

Known fault detectors further require a full half-cycle of fault currentto respond thereto. During the time required to detect a fault, costlyequipment may be damaged. Additional problems arise in locating thefault after it has been cleared by the transmission system'sinterrupting devices.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages of priorfault detectors have been overcome.

The fault detector includes a remote current sensor for measuring themagnetic field surrounding a conductor of an electrical transmissionsystem to monitor the current flowing through the conductor. The currentsensor is positioned at a remote location from the conductor. That is ata location sufficiently distant from the conductor to prevent electricalarcing from occurring without the use of an insulating material.

The remote current sensor includes a Hall effect transducer formeasuring the magnetic field surrounding the conductor. The transduceris positioned between a pair of tapered pole pieces having a widecross-sectional area at one end and a small cross-sectional area at theopposite end, adjacent to the transducer. The tapered pole piecesconcentrate low density magnetic flux in the air into a high densitymagnetic flux focused onto the transducer enabling the detection of amagnetic field from a distance which is greater than the electricalarcing distance.

The conductors of a three phase electrical transmission system may bearranged in either a triangular configuration or a coplanarconfiguration. For a triangular configuration of conductors, two remotecurrent sensors are utilized. A first remote current sensor ispositioned so that its sensitive axis lies in the plane of the first andsecond conductors and is tangent to the magnetic field surrounding thethird conductor. So positioned, the first current sensor measures themagnetic field due to the current flowing through the third conductorand is insensitive to the fields around the other conductors. The secondremote current sensor is positioned so that its sensitive axis isperpendicular to the sensitive axis of the first remote current sensorand tangent to the magnetic fields surrounding the first and secondconductors. So positioned, the second current sensor measures the vectorsum of the magnetic fields due to the currents flowing through the firstand second conductors and is insensitive to the field around the thirdconductor.

For a triangular configuration of conductors wherein the altitude isequal to one-half of the base, both of the current sensors arepositioned at a location equidistant from all three conductors. Thisenables each of the sensor outputs to be compared to a single set pointin determining the existence of a fault on the transmission line.

For a coplanar configuration of conductors, three remote current sensorsare utilized. Each of the remote current sensors is positioned radiallyoutward from its associated conductor, the sensitive axis of each sensorbeing tangent to the magnetic field surrounding its conductor.

Each of the remote current sensors is coupled to an amplifier through acapacitor which eliminates the effects of constant magnetic fields fromthe sensor output signals. A high signal selector selects the mostpositive amplified sensor signal to be compared with a first set pointin order to detect a positive polarity fault current. A low signalselector selects the least positive amplified sensor signal to becompared with a second set point in order to detect a negative polarityfault current. Because the fault detector recognizes sensor signalscorresponding to both positive and negative polarity fault currents, thedetector can respond to a fault within one quarter of the fault currentcycle.

If either of the first or second set points is exceeded by the amplifiedsensor signal, a relay is actuated to provide a fault indication signal.Means are provided to prevent the actuation of the relay due totransient signals which may arise when the fault detector power supplyis lost or subsequently restored.

Further advantages of the invention will be readily apparent from thefollowing specification and from the drawings, in which:

FIG. 1 is a cross-sectional view of a remote current sensor used in thefault detector of the present invention;

FIG. 2A illustrates the positioning of two current sensors used with atriangular configuration of conductors;

FIG. 2B illustrates the positioning of three current sensors used with acoplanar configuration of conductors;

FIG. 3 is a cross-sectional view of the housing for the current sensorsused with a triangular configuration of conductors;

FIG. 4 is a cross-sectional view of a housing for an individual currentsensor used with a coplanar configuration;

FIG. 5 is a block diagram of the fault detector with two currentsensors;

FIG. 6 is a graphical illustration of a fault current; and

FIG. 7 is a schematic diagram of the fault detector circuit with threecurrent sensors.

SPECIFICATION

The fault detector of the present invention utilizes a remote currentsensor as shown in FIG. 1 to measure the magnetic field surrounding aconductor of an electrical transmission system and thereby monitor thecurrent flowing through the conductor. The current sensor is positionedat a remote location from the conductor. That is, at a location adistance from the conductor which is greater than the electrical arcingdistance so as to eliminate the need for expensive insulating materials.

The remote current sensor includes a Hall effect transducer 10 formeasuring the magnetic field surrounding a conductor. The Hall effecttransducer should have high sensitivity, linearity, repeatability, andstability over a wide range of ambient temperatures. The Honeywell92SS12-2 transducer is suitable. The magnetic field surrounding aconductor is proportional to i/d where i is the current flowing throughthe conductor and d is the distance between the conductor and the pointof measurement. The output of the Hall effect transducer 10 is a voltageproportional to the magnetic field sensed and thus proportional to thecurrent flowing through the conductor. The output voltage is provided bytransducer 10 on lines 12 to the fault detector circuit for suchpurposes as identifying and locating fault currents, controlling theoperation of circuit breakers, and accumulating operational data.

The remote current sensor is located at a distance from the conductorwhich is greater than the electrical arcing distance so as to allow useof the air gap between the conductor and the remote current sensor asinsulation. For a 15,000 volt transmission line, the remote currentsensor is located approximately 20" from the conductors and for a138,000 volt line, the sensor is located approximately 60" from theconductors. These distances are sufficiently remote from the conductorsto prevent electrical arcing from occurring without the use of aninsulation material.

At such distances from the conductors, the magnetic flux density in theair is low and existing Hall effect transducers alone cannot accuratelymeasure the magnetic field. In order to detect the low density magneticflux at a remote location from the conductors, the Hall effecttransducer 10 is positioned between a pair of pole pieces 14 and 16,made of a low hysteresis ferromagnetic material. The pole pieces 14 and16 are tapered having a wide cross-sectional area at respective ends 18and 20 and a small cross-sectional area at the opposite ends 22 and 24adjacent to the transducer 10. The wide pole piece ends 18 and 20collect low density magnetic flux in the air and focus the flux into aconcentrated high density magnetic field between the small pole pieceends 22 and 24. The resulting magnetic field focused onto the Halleffect transducer is proportional to the magnetic field in the air buthas a much higher density to enable the detection of the field from aremote location.

The Hall effect transducer 10 is mounted in a flexible silastic pottingmaterial 25 through the inner wall of a transducer sleeve 26. A sleeve28 rigidly supports the pole pieces 14, 16 and the transducer sleeve 26so that the magnetic field sensed is focused directly onto the sensitivearea 30 of the transducer 10. The sleeve 28 is made of fiberglass orother insulating material in order to eliminate eddy currents whichwould arise if the sleeve were made of a conducting material. Eddycurrents will produce an erroneous transducer output so that it isessential that such currents be eliminated.

The three phase conductors of an electrical transmission system may bearranged in either a triangular configuration as shown in FIG. 2A or ina coplanar configuration as shown in FIG. 2B. For a triangularconfiguration, overhead conductors 32, 34 and 36 are secured torespective insulators 38, 40 and 42 mounted on a box-like housing 44which is supported by a pole 46. The box-like housing 44 may containdisconnect switches for the conductors as shown in Bridges U.S. Pat. No.4,095,061. A current detector generally designated 48 includes tworemote current sensors 50 and 52. The current detector 48 is mounted ona base 54 of the box-like housing 44. So mounted, the current sensorsare at a sufficiently remote location from the conductors to preventelectrical arcing from occurring without the use of an insulatingmaterial.

The remote current sensors 50 and 52 of the current detector 48 aredisposed within an electrically grounded cylindrical housing having aheavy aluminum wall 56 as shown in FIG. 3. The current sensor 50 issecured to the aluminum wall 56 by a screw 58 connected to a pole pieceend 60. The current sensor 52 is secured to the aluminum wall 56perpendicular to sensor 50 by a screw 62 connected to a pole piece end64.

The aluminum housing of the current detector 48 is mounted on the base54 so that the sensitive axis 66 of the remote current sensor 50 lies inthe plane of conductors 32 and 36 and is tangent to the circularmagnetic field 68 surrounding conductor 34. The remote current sensor 52is positioned so that its sensitive axis 70 is perpendicular to thesensitive axis of sensor 50 and is tangent to the magnetic fields 72 and74 surrounding conductors 32 and 36 respectively.

Only that field which is parallel to the sensitive axis of a remotecurrent sensor is detected by the sensor so that sensor 50 measures themagnetic field surrounding conductor 34 and sensor 52 measures themagnetic fields surrounding conductors 32 and 36. Since the sensors 50and 52 have orthogonal sensitive axes, each sensor is insensitive to themagnetic field detected by the other sensor. Sensor 50 is insensitive tothe magnetic fields surrounding conductors 32 and 36, these fieldsintersecting the sensitive axis of sensor 50 at right angles since thesensitive axis of sensor 50 lies in the plane of conductors 32 and 36.Similarly, the magnetic field surrounding conductor 34 intersects thesensitive axis of the sensor 52 at right angles so that sensor 52 isinsensitive to that field.

Since only one remote current sensor is used to measure the magneticfields surrounding both of the conductors 32 and 36, sensor 52 should bepositioned equidistant from both of the conductors 32 and 36 so as to beequally sensitive to the respective magnetic fields surrounding theconductors. For a triangular configuration of conductors having analtitude equal to one-half of the base as shown in FIG. 2A, both of theremote current sensors 50 and 52 are positioned equidistant from allthree of the conductors 32, 34 and 36 so that the outputs of the sensorswill be of the same order of magnitude. Further, fault currents are of asufficiently higher magnitude than are load currents to enable theoutputs of both of the sensors 50 and 52 to be compared to the same setpoint in determining the existence of a fault to be discussed in detailbelow in connection with the fault detector block diagram.

The coplanar configuration of conductors as shown in FIG. 2B utilizesthree remote current sensors 76, 78 and 80 associated with respectiveconductors 82, 84 and 86. Each of the remote current sensors is mountedin an individual aluminum housing 88 as shown in FIG. 4 for sensor 76.The remote current sensor 76 is secured to the aluminum housing 88 by ascrew 90 connected to a pole piece end 92. A steel pipe 94 isthreadingly connected to the housing 88 and contains the leads 96 fromthe Hall effect transducer of sensor 76 connected to the fault detectorcircuit.

Each of the remote current sensors 76, 78 and 80 is positioned radiallyoutwardly from its associated conductor, the sensitive axis of eachsensor being tangent to the magnetic field surrounding its conductor.The remote current sensors are located at a distance from the conductorswhich is greater than the electrical arcing distance. Such distancesallow the aluminum housing 88 for each sensor to be electricallygrounded so that the air gap separating the sensors from the conductorsmay be used as insulation. Further, the distances separating each of theremote current sensors from their respective conductors should be equalso that the outputs of all three sensors may be compared to the same setpoint in determining the existence of a fault.

Although the remote current sensor 76 predominantly measures themagnetic field surrounding the conductor 82, the sensor is also affectedby the magnetic fields due to the currents flowing through theconductors 84 and 86. Similarly, sensors 78 and 80 are affected by themagnetic fields due to the currents flowing through conductors otherthan their respectively associated conductors 84 and 86. The effects ofthe currents flowing through the other conductors on a particular remotecurrent sensor may be compensated for by adjusting the set point in thefault detector to be discussed in detail below.

A block diagram of the fault detector is shown in FIG. 5 having tworemote current sensors 50 and 52 as would be used with a triangularconfiguration of conductors. The block diagram is equally applicable toa fault detector used with a coplanar configuration of conductors. Theonly difference being the addition of a third remote current sensor andassociated amplifier.

In order to provide an accurate measurement of the magnetic fields dueto the currents flowing through the conductors of an electricaltransmission system, the effects of the earth's magnetic field must beeliminated from the output signals of the remote current sensors 50 and52. The earth's magnetic field is relatively constant as opposed to themagnetic field surrounding a conductor of an electrical transmissionsystem which varies at a 60 cycle rate. Capacitors 100 and 102 coupledto the respective outputs of sensors 50 and 52 effectively eliminate theD.C. component of the sensor signal attributable to a constant magneticfield such as the earth's field. Only the A.C. components of the sensorsignals are passed by capacitors 100 and 102 to respective amplifiers104 and 106 so that the amplified signals accurately reflect themagnetic field due solely to the current flowing through the conductors.

The outputs of amplifiers 104 and 106 are the amplified A.C. sensorvoltage superimposed upon a steady state D.C. voltage. Depending uponthe value at which the steady state D.C. voltage of the amplifier isset, the negative half-cycle of the amplified sensor signal may have apositive value. Therefore, a reference to a negative half-cycle of theamplified sensor signal connotes a negative half-cycle with respect tothe steady state D.C. voltage of the amplifier output.

One application of the fault detector is the identification and locationof a fault current as shown in FIG. 6. A fault current may result from ashort circuit between a conductor and ground or between any two or allthree conductors. Interrupting devices in the transmission line willtypically clear a fault in one half-cycle of a 60 cycle power frequencyto prevent the high currents from damaging costly equipment. In order toprovide a fault indication so that the fault may be located before beingcleared, the fault detector recognizes both positive and negativepolarity fault currents within one quarter of a cycle.

A high signal selector 108 selects the higher positive amplified sensorsignal from either of amplifiers 104 or 106 and passes it to acomparator 110. If the higher positive amplified sensor signal exceeds aset point which is input to the comparator 110 on a line 112, then theoutput of comparator 110 goes high, indicating a fault occurring duringthe positive half-cycle of the current. A low signal selector 114selects the least positive amplified sensor signal to be passed to acomparator 116. If the least positive amplified sensor signal is lessthan a set point which is input to the comparator 116 on a line 118, theoutput of comparator 116 goes high, indicating a fault occurring duringthe negative half-cycle of current. Since faults occurring during boththe positive and negative half-cycles of current are recognized byrespective comparators 110 and 116, the fault detector can provide afault indication within approximately one quarter of the fault currentcycle before the fault is cleared by an interrupting device.

It is noted that the higher positive amplified sensor signal is comparedto a single set point, which is input to comparator 110 on line 112,regardless of which of the sensors 50 or 52 the signal originates from.Similarly, a single set point is compared to the least positiveamplified sensor signal by the comparator 116 regardless of which of thesensors 50 or 52 the signal originates from. As discussed in connectionwith FIG. 2A for a triangular configuration of conductors, the fluxconditions on the remote current sensor 50 and 52 are different. Due tothe orthogonal orientation of the sensitive axes of the sensors 50 and52, sensor 50 detects only the magnetic field surrounding conductor 34,whereas sensor 52 detects the vector sum of the magnetic fieldssurrounding conductors 32 and 36. Even though the flux conditions oneach of the sensors 50 and 52 are different, a single set point may beused for comparison to either of the amplified sensor signal outputsbecause of the specific arrangement of the sensors with respect to theconductors as shown in FIG. 2A.

For a triangular configuration of conductors having an altitude which isequal to one-half of the base, sensors 50 and 52 are both positionedequidistant from all three of the conductors 32, 34 and 36. Sincesensors 50 and 52 are positioned equidistant from the conductors, eventhough the magnetic flux conditions in each of the sensors aredifferent, the outputs of sensors 50 and 52 will be of the same order ofmagnitude. Fault currents are of a sufficiently higher magnitude thanare load currents so that a sensor output corresponding to a loadcurrent may be distinguished from a sensor output corresponding to afault current using a single set point for comparison with the outputsfrom either of the sensors 50 or 52. A set point adjustment 120 isadjusted so that the maximum output from sensor 50 corresponding to aload current flowing through the conductor 34 will be ignored bycomparators 110 and 116, but the minimum output from sensor 52corresponding to a line-to-ground fault on either of conductors 32 and36 will be detected.

For a coplanar configuration of conductors, the distances separatingeach of the remote current sensors from their respective conductors areequal so that the outputs of each of the remote current sensors 76, 78and 80 will be of the same order of magnitude to enable use of a singleset point. In adjusting the set point adjustment 120 for use with acoplanar arrangement of conductors, the vector sum of the magneticfields detected by a single sensor should be determined to account forthe effects of currents flowing through all three conductors on thesensor.

When either of the outputs of comparators 110 or 116 goes highindicating a fault, a high signal selector 122 actuates a switchingtransistor 124. When turned on, the switching transistor 124 conducts,drawing current through a high speed relay 126, thereby setting therelay. When relay 126 is set, its contacts 127 close to provide a faultindication signal on lines 128. The fault indication signal may betransmitted on lines 128 to a supervising station which monitors theelectrical transmission system. At the supervising station, the faultindication signal can be used to locate the fault or it can be used toaccumulate operational data. The fault indication signal provided online 128 may also be used to actuate circuit breakers in order toprotect the equipment of the electrical transmission from damaging faultcurrents.

In order to reset the relay 126 after a fault has been identified, arelay contact 129 is closed causing current to flow through the relay ina direction opposite to the direction of current which sets the relay.

FIG. 7 is a detailed schematic of the fault detector block diagram ofFIG. 5 having an additional remote current sensor. For a coplanarconfiguration of conductors, three remote current sensors 76, 78 and 80are used having associated amplifier circuits 130, 132 and 134. Whereonly two remote current sensors are used as in conjunction with atriangular configuration of conductors, the only difference in the faultdetector circuit is the elimination of one of the amplifier circuits.

The amplifier circuits associated with each of the remote currentsensors are identical so that only the amplifier circuit 130 associatedwith sensor 76 will be discussed. A 15+V power supply provides the Halleffect transducer of the remote current sensor 76 with the properbiasing voltage on a line 136, line 138 being connected to ground. Thetransducer output voltage appears across a resistor 140 and isproportional to the magnetic field surrounding the conductor 82 and thusproportional to the current flowing through the conductor. A capacitor142 provides A.C. coupling to an amplifier 144, the D.C. component ofthe sensor signal associated with the earth's constant magnetic fieldbeing eliminated thereby. The gain of amplifier 144 is determined by theratio of a feedback resistor 146 to the value of a resistor 148. Thesteady state D.C. output voltage of amplifier 144 is set by the valuesof the resistor 148 and a bias resistor 150. In the presence of analternating magnetic field, the output of the remote current sensor 76is an A.C. voltage which is input to the noninverting terminal 152 ofamplifier 144. The output of amplifier 144 is the amplified A.C. voltagefrom sensor 76 superimposed upon the steady state D.C. output voltage ofamplifier 144.

Each of the amplified sensor signals which is output from amplifiers144, 154 and 156 is applied to a respective pair of oppositely poleddiodes 158, 159; 160, 162; and 164, 166. Diodes 158, 160 and 162 haveanodes connected to the outputs of the amplifiers and cathodes tiedtogether by line 168 connected to ground through a resistor 170 actingas the high signal selector 108 to pass the highest positive amplifiedsensor signal to the noninverting input terminal of comparator 110through a resistor 172. The diodes 159, 162 and 166 have cathodesconnected to the outputs of respective amplifiers 144, 154 and 156 andanodes tied together by line 174 connected to the +15 V power supplythrough a resistor 176 acting as the low signal selector 114 to pass theleast positive amplified sensor signal to the inverting input terminalof the comparator 116 through a resistor 178.

Comparator 110 compares the highest positive amplified sensor voltage,selected by diodes 158, 160 and 164, to a set point voltage applied tothe inverting input terminal of the comparator through a resistor 180.The set point voltage is taken from the positive end of a potentiometer182 connected to the +15 V power supply through a resistor 184. If thehighest positive amplified sensor voltage is greater than the set point,the output of comparator 110 goes high indicating a positive polarityfault current.

Comparator 116 compares the least positive amplified sensor voltage,selected by diodes 159, 162 and 166, to a set point voltage applied tothe noninverting input terminal of the comparator through a resistor186. The set point voltage is taken from the negative end ofpotentiometer 182 connected to ground through a resistor 188. If theleast positive amplified sensor voltage is less than the set point, theoutput of comparator 116 goes high indicating a negative polarity faultcurrent. The potentiometer 182 is adjusted so that the set pointvoltages applied to the comparators 110 and 116 will only be exceeded byan amplified sensor voltage indicating a fault current as discussed inconhection with FIG. 5.

The outputs of comparators 110 and 116 are connected to the base of aswitching transistor 190 through a resistor 192 and respective zenerdiodes 194 and 196. The zener diodes act as the high signal selector 122and select the higher of the outputs from comparators 110 and 116 to beapplied to the base of transistor 190. When load currents flow throughthe conductors of an electrical transmission system, the amplifiedsensor voltages are such that the highest positive amplified voltage isless than the set point voltage applied to comparator 110 and the leastpositive amplified voltage is greater than the set point voltage appliedto comparator 116. Under these conditions, the outputs of comparators110 and 116 are saturated at +1 V. Because of a 6 V drop across each ofthe zener diodes 194 and 196, the normally low comparator outputs areinsufficient to turn on the switching transistor 190. However, when afault current occurs, the set point voltage will be exceeded in eitherthe positive or negative sense by the highest positive amplified sensorvoltage or the least positive amplified sensor voltage so that theoutput of the respective comparator jumps from +1 V to +14 volts turningon transistor 190. The collector of transistor 190 is connected to the+15 V power supply through a resistor 198 and the emitter of transistor190 is connected to ground through a second transistor 200 to bedescribed in detail below. If transistor 200 is turned on, a high outputfrom either of comparators 110 or 116 turns on transistor 190 causingthe transistor to conduct through transistor 200.

A magnetic latching reed relay 202 is connected between the +15 V powersupply and the collector of transistor 190 by means of switches 204 and206. The switches 204 and 206 provide for the selection of a normallyopen relay or a normally closed relay. When switches 204 and 206 are inthe position shown contacting terminals 208 and 210 respectively, thenegative terminal 212 of relay 202 is connected between the collector oftransistor 190 and the 15 V power supply through the resistor 198, thepositive terminal 216 of the relay being connected to the +15 V powersupply through a resistor 218. Relay contact 220 is normally open. Iftransistors 190 and 200 are turned on indicating the detection of afault current, transistor 190 conducts through transistor 200 therebydrawing current through relay 202 in the direction to set the relay andclose the relay contact 220. When the relay contact 220 is closed, afault indication signal is provided on lines 222 to a supervisingstation for the electrical transmission system. The supervising stationmay use the fault indication signal to locate the fault on thetransmission line or to accumulate operational data for the system.

When switches 204 and 206 contact terminals 224 and 226, the negativeterminal 212 of relay 202 is connected to the +15 V power supply throughthe resistor 218 and the positive terminal 216 of the relay is connectedbetween the collector of transistor 190 and the +15 V power supplythrough the resistor 198. The relay contact 220 is normally closed. Whenthe transistor 190 conducts through transistor 200, current will bedrawn through relay 202 in the direction to open the relay contact 220.With switches 204 and 206 contacting terminals 224 and 226 so that therelay contact 220 is normally open, the relay may be used to controlinterrupting devices which open upon detection of a fault current.

Once relay 202 is set, the relay contact 220 will remain latched in aclosed (or open) position until the fault indication signal isacknowledged by resetting relay 202. A reset contact 228 is connectedbetween the +15 V power supply through the resistor 218 and ground. Whencontact 228 is momentarily closed, current passes through relay 202 in adirection opposite to the direction of current which sets the relaythereby opening (or closing) the relay contact 220.

The Hall effect transducer used in the remote current sensor has aregulated output which is independent of a power supply of +8 V D.C. orgreater. However, when the power supply is below 8 volts, transientsensor signals result. When the power supply is lost, the output of theHall effect transducer drops, producing a transient sensor signal of thesame order of magnitude as a sensor signal indicating a negativepolarity fault current. Similarly, when the power supply is restored,the output of the transducer rises until the power supply voltage hasincreased to +8 V, producing a transient sensor signal of the same orderof magnitude as a sensor signal indicating a positive polarity faultcurrent. In order to prevent an erroneous fault indication due to atransient sensor signal, the actuation of transistor 200 is delayedthereby preventing trahsistor 190 from conducting and setting the relay202.

A time delay amplifier 230 is connected between the +15 V power supplythrough a resistor 232 and the base of the transistor 200 through aresistor 234. The appropriate time delay is provided by a capacitor 236connected between the inverting input terminal of amplifier 230 andground and a feedback circuit comprised of a feedback resistor 238 inseries with the parallel combination of a resistor 240 and a capacitor242. When the +15 V power supply is lost, the output of the amplifier230 drops sufficiently low to turn off transistor 200 before a transientsensor signal can turn on transistor 190, thereby preventing theactuation of relay 202. When the +15 V power supply is restored, theoutput of the amplifier 230, a ramped voltage rising to +2 V withinapproximately 2 seconds, delays the actuation of the transistor 200. Thedelay is sufficient to prevent transistor 190 from conducting throughtransistor 200 and actuating the relay 202 due to a transient sensorsignal.

The power supply for the fault detector may be powered by thetransmission line which the fault detector is monitoring. A fault on thetransmission line may cause the loss of the power supply at the instantit is essential that the fault detector be operating to identify andlocate the fault. In order to maintain the D.C. output voltage of thepower supply above the minimum level required for the detector tooperate, the power supply should include sufficient storage capacity.Large filter capacitors in the power supply will provide a slow rate ofdecay of the D.C. voltage to allow the fault detector to operate for atime sufficient to provide a fault indication to the supervisory programbefore operating power is completely lost.

I claim:
 1. A remote current sensor for monitoring the current flowingthrough a conductor, said sensor to be positioned in a magnetic fieldsurrounding the conductor, comprising:a Hall effect transducer formeasuring the magnetic field surrounding said conductor; a pair oftapered pole pieces with said transducer positioned therebetween, eachof said pole pieces being made of a ferromagnetic material and having awide cross sectional area at one end to pick up a low density magneticflux in the air and a small cross sectional area at the opposite endadjacent to the transducer to concentrate said low density magnetic fluxinto a high density magnetic flux focused onto the transducer, thetransducer and pole pieces being positioned at a location removed fromthe conductor greater than the electric arcing distance for the voltageat which the conductor is normally operated; and a grounded aluminumhousing for said sensor to shield the sensor from electric fields. 2.The remote current sensor of claim 1 further including a sleeve rigidlysupporting said pole pieces so that the magnetic field picked up by thepole pieces is focused directly onto a sensitive area of saidtransducer.
 3. The remote current sensor of claim 2 wherein said sleeveis of an electrically insulating material.